Implantable cardiac devices are commonplace today, particularly for treating cardiac rhythm dysfunction. Cardiac pacemakers, for example, are implantable medical devices that delivering electrical pacing pulses to the heart in order to treat bradycardia (a heart rate that is too slow) due to either chronotropic incompetence or a conduction system defect or to treat tachycardia (a heart rate that is too fast). Implantable cardioverter/defibrillators (ICD's) are devices that deliver electrical energy to the heart in order to reverse excessively rapid heart rates including life threatening cardiac arrhythmias such as ventricular fibrillation. Since some patients have conditions that necessitate pacing and also render them vulnerable to life-threatening arrhythmias, implantable cardiac devices have been developed that combine both functions in a single device. Also included within the concept of cardiac rhythm is the manner and degree to which the heart chambers contract during a cardiac cycle to result in the efficient pumping of blood. Patients who exhibit pathology of conduction pathways, such as bundle branch blocks, can suffer compromised cardiac output. In order to treat these problems, pacemakers have been developed which provide electrical pacing stimulation to one or both of the atria and/or ventricles during a cardiac cycle in an attempt to improve the coordination of atrial and/or ventricular contractions, termed cardiac resynchronization therapy.
Most pacemakers today are operated in some sort of synchronous mode where the pacing pulses are delivered in a manner that is dependent upon the intrinsic depolarizations of the heart as sensed by the pacemaker. ICD's must also sense the electrical activity of the heart in order to detect an arrhythmia that will trigger delivery of the shock pulse in an attempt to reverse the condition. Such sensed information can also be stored by the device and transferred later to an external programmer via a radio link. The present invention is concerned with a method and system by which an implantable cardiac device may store data representing the electrical activity of the heart for display by an external programmer or other apparatus in a clinically useful manner.